There has hitherto been known an exothermic body containing CaO as a main ingredient for heating a cooked food contained in a flat bag. This exothermic body utilizes heat generated by an exothermic reaction of CaO with water to heat the food.
However, the above-described exothermic body has entailed a danger of setting a flammable material on fire or burning a person due to damage of a recipient (enveloping body) containing the exothermic body because it rapidly generates a great amount of heat upon occurrence of the exothermic reaction.
For this reason, there has been earnestly desired a safe heat source free of such a disadvantage.
In view of the above-mentioned circumstance, it is an object of the present invention to provide a safe heat source which avoids rapid generation of a great amount of heat and is capable of easily heating a food or the like.